This invention is in the field of thermoplastic fluoropolymers, more particularly crosslinked thermoplastic fluoropolymers.
Thermoplastic fluoropolymers are well known for outstanding combinations of properties including chemical resistance, unique surface characteristics, high service temperatures, and good dielectric characteristics. As a result, fluoropolymer resins are used in a wide variety of applications including wire insulation, cable jacket, hose, tubing, film, linings for chemical process equipment, articles for fluid handling in laboratory and manufacturing situations and the like. The service temperature in some of these applications can be high. As is common for thermoplastics, some properties of fluoropolymers change as temperature increases. Modulus and tensile strength, for example, typically decrease with increasing temperature.
Efforts have been made to improve the physical characteristics of fluoropolymers at elevated temperatures, largely by cross-linking. Approaches to cross-linking usually involve the incorporation of a cross-linking promoter, also called,a coagent, such as triallyl cyanurate or triallyl isocyanurate (U.S. Pat. No. 4,353,951) or metallic diacrylate (U.S. Pat. No. 5,409,997) into a fluoropolymer such as ETFE copolymer, followed by treatment with ionizing radiation to effect the cross-linking.
Improved ways to cross-link fluoropolymers and thereby achieve improved properties are desired.
This invention provides a cross-linked fluoropolymer, the fluoropolymer being thermoplastic and melt-fabricable prior to cross-linking, wherein the cross-linking of the fluoropolymer is carried out thermally.
The invention also provides a process for cross-linking a melt-fabricable thermoplastic fluoropolymer having pendant functional groups, comprising combining the fluoropolymer with cross-linking promoter before melt fabrication of the fluoropolymer into a shaped article is completed, and then completing melt fabrication. The fluoropolymer can also be shaped into an article before exposure to the cross-linking promoter, which is then infused into the article. Preferred cross-linking promoters are nucleophiles, and include monofunctional and polyfunctional amines.
It has been discovered that functionalized fluoropolymer can be effectively cross-linked by a thermal process that does not require the costly use of ionizing radiation and the expensive equipment necessary for such treatment. Functionalized fluoropolymer that can be so cross-linked has pendant functional groups and is melt-fabricable prior to cross-linking. The cross-linking takes place in the presence of cross-linking promoter. Since the cross-linking occurs thermally, the cross-linking process can be coordinated with the fabrication of the melt-fabricable fluoropolymer into a shaped article.
As used herein, xe2x80x9cthermal processxe2x80x9d and xe2x80x9coccurs thermallyxe2x80x9d are understood to mean that the cross-linking process of the invention is activated by temperature alone. It suffices that functionalized fluoropolymer and cross-linking promoter are brought together and intermixed, and the temperature is raised to at least a level sufficient to activate cross-linking for a time sufficient to achieve the desired cross-linking. One skilled in the art will recognize that the time required to achieve cross-linking will in general depend on the temperature, with cross-linking occurring more rapidly as temperature increases. Typically, a temperature of at least 75xc2x0 C. and more often at least 100xc2x0 C. will be required for cross-linking to occur to any appreciable extent. Thus, cross-linking initiation as might be provided by such means as a catalyst or by ionizing radiation is not necessary. One skilled in the art will recognize the possibility that the thermal cross-linking of the invention could be supplemented by such auxiliary means. Such supplemented cross-linking processes are within the scope of the present invention if cross-linking occurs in the absence of such auxiliary means.
The cross-linking promoter used in the thermal cross-linking of the instant invention is present in minor amount. When the cross-linking promoter is a low molecular weight compound, such as a compound having a molecular weight of 500 or less, the amount of cross-linking promoter will generally be no more than 2 wt % based on the weight of fluoropolymer resin, usually no more than 1 wt %. The amount of such cross-linking promoter present is generally at least 0.01 wt %, more commonly at least 0.05 wt %, based on weight of fluoropolymer. When cross-linking promoter is polymeric, then larger amounts of cross-linking promoter can be used, such as 3-30 wt % based on combined weight of fluoropolymer and cross-linking promoter, more commonly 10-25 wt %. Cross-linking promoter should be stable by itself at processing temperatures.
Cross-linking promoters that can be used include nucleophiles, including monofunctional and polyfunctional nucleophiles. Preferred nucleophiles include amines, including primary and secondary amines. Polyfunctional amines that can be used include, for example, aromatic diamines such as para-phenylenediamine, meta-phenylenediamine and methylene dianiline. aliphatic diamines such as 1,6-diaminohexane, and aliphatic triamines such as bis(hexamethylene)triamine. Monofunctional amines that can be used include, for example, aniline, diethyl amine and ammonia. The use of monofunctional cross-linking promoters is particularly surprising. Polymeric cross-linking promoters that can be used in the present invention include polyamides. One skilled in the art will recognize that more than one cross-linking promoter can be used. Thus, at least one cross-linking promoter is used.
Since the cross-linking of the present invention occurs thermally, it is difficult to provide a cross-linkable composition, containing fluoropolymer having pendant functional groups and cross-linking promoter, in conventional cube form because such cubes are normally prepared by melt extrusion, i.e., a high-temperature process which would promote the thermal cross-linking and render the composition less suitable or unsuitable for subsequent fabrication into desired articles or shapes. Additionally, some care must be exercised so that thermal cross-linking does not take place too rapidly and/or to too great a degree within melt processing equipment, e.g., an extruder or an injection molding machine, and thereby impede or even prevent the flow of resin within or out of the equipment. Cross-linking promoter and fluoropolymer having pendant functional groups can first be combined in desired proportions at room temperature, and the mixture then shaped by the intended melt fabrication technique, in which case the cross-linking temperature will usually be at least 250xc2x0 C. and more often at least 275xc2x0 C. Promoter and fluoropolymer can be blended with each other in the dry state, such as by tumbling in a drum, or can be combined by simultaneous or separate metering of the feed of one or more of the components to the melt processing device. As illustrated by examples below, low molecular weight cross-linking promoter can also be deposited on fluoropolymer resin from a solution of promoter in an appropriate solvent. Preliminary combining of promoter and fluoropolymer can be satisfactory if residence time in the melt processing equipment is relatively short, but may not be satisfactory if residence time is relatively long or cross-linking is relatively rapid. Promoter and functionalized fluoropolymer can also be combined during melt processing in a way that reduces residence time of the melt-processible composition of the invention in the melt processing equipment, thereby reducing or avoiding the risk of undesired melt viscosity (MV) increase inside the equipment. This can be accomplished, for example, by injecting promoter, such as in a solution, into the melt of fluoropolymer having pendant functional groups as it (the melt) is transported along the barrel of a melt extruder equipped with means for injecting fluid substances. In such a procedure, promoter can be injected at a point sufficiently upstream from the melt exit (the die) to achieve good mixing, but also sufficiently close to the die that any cross-linking that may occur inside the extruder does not impede melt exit from the extruder.
When polyamide is used as cross-linking promoter, fluoropolymer and polyamide are preferably melt-blended together under high shear. The ingredients can first be combined in desired proportions and blended with each other in the dry state, such as by tumbling in a drum, or can be combined by simultaneous or separate metering of the, feed of one or more of the components to the melt blending device. Preferably, the melt blending is done in a twin screw extruder, such as manufactured by Werner and Pfleiderer or by Berstorff. Numerous other high shear melt blending devices, as known to those skilled in the art, can be used without departing from the spirit of the invention. When polyamide is used as cross-linking promoter, the fluoropolymer is the continuous phase of a melt blend and the polyamide is present in the fluoropolymer as a dispersed phase.
The composition of the present invention can contain additives such as commonly used in thermoplastics, including stabilizers, pigments, fillers, e.g., glass or graphite, and the like. Such additives are xe2x80x9cinertxe2x80x9d in the sense that they do not participate in the cross-linking in an essential way.
Cross-linking can be reflected by changes in various properties. Typically, melt viscosity, and tensile strength increase, while tensile elongation at break decreases. Hardness and or flex modulus can also increase to indicate that cross-linking has occurred. For crosslinking of the present invention, substantial increases in flex modulus have been observed. See Example 2, which discloses an increase of more than 100%. Such large increases are not required to conclude that cross-linking has taken place. For example, an increase of 25%, or of 10%, would indicate that cross-linking has occurred. Alternatively or additionally, cross-linking of fluoropolymer having pendant functional groups in the presence of cross-linking promoter is indicated by a decrease of melt flow rate (MFR) (increase of melt viscosity). Again, see Example 2 which shows a decrease of MFR to zero. As with flex modulus, such large changes are not required to conclude that cross-linking has occurred. For example, a decrease in MFR of 30%, or even of 15%, would indicate that cross-linking has occurred.
The fluoropolymer resin in the composition of the invention is melt-fabricable, and, thus, can be converted to shaped articles by melt processing techniques such as extrusion, injection molding, compression molding, transfer molding, and the like. For fabrication by such techniques, melt viscosity (MV) is usually in the range of 0.5xc3x97103 to 100xc3x97103 Paxc2x7s as conventionally measured for the particular fluoropolymer, though MV outside this range is known. Preferably, MV is in the range of 1-25xc3x97103 Paxc2x7s. If the fluoropolymer resin is a blend of fluoropolymers, the MV of each fluoropolymer component is usually within the aforesaid ranges, but blending can permit components to have MV in a wider range, as will be understood by one skilled in the art. For fluoropolymers, MV is calculated from melt flow rate (MFR) measured according to a modification of ASTM D-1238. For example, see U.S. Pat. No. 4,380,618 and the ASTM Standards cited below for specific fluoropolymers. MV is inversely related to MFR by an expression that depends on the test conditions and the density of the polymer.
As used herein, xe2x80x9cfunctionalized fluoropolymerxe2x80x9d means fluoropolymer having functional side groups or functional groups attached to side groups, i.e., pendant functional groups. Usually, but not necessarily, such functional units are at the ends of the pendant side groups. Fluoropolymer that does not have such pendant functional groups is sometimes described herein as xe2x80x9cnon-functional fluoropolymerxe2x80x9d. Thus, non-functional fluoropolymer and functionalized fluoropolymer differ at least by the presence in the latter of pendant functional groups. Non-functional fluoropolymer can be a precursor to functionalized fluoropolymer, in which instance the process of functionalizing involves addition of functional groups to the non-functional polymer. However, xe2x80x9cfunctionalizingxe2x80x9d is also used in a broader sense herein to include preparation of functionalized fluoropolymer which would be non-functional if pendant functional groups were not present, even though non-functional fluoropolymer may not be the precursor.
Functional groups, in the context of the present invention, are groups capable of participating in cross-linking reactions, when functional groups and cross-linking promoter are both present in a fluoropolymer composition. Such functional groups can be introduced, for example, by incorporating into the fluoropolymer, during polymerization, monomer units having such functional groups, i.e., functional monomers, or by having a compound grafted thereto which imparts polar functionality to the fluoropolymer. Such grafted fluoropolymer includes the grafted fluoropolymer powder described in U.S. Pat. No. 5,576,106 and the grafted fluoropolymer described in EP 0 650 987. Other known methods of grafting can be used. Preferred polar-grafted fluoropolymers include the surface-grafted powder of the ""106 patent. Examples of polar functionality provided by grafting include acids, including carboxylic, sulfonic and phosphonic acids, and esters and salts thereof, and epoxides. Glycidyl methacrylate is an example of a grafting compound that provides epoxide functionality. Among compounds for grafting onto and thereby becoming part of the polar-grafted fluoropolymer, maleic acid and maleic anhydride (MAnh) are preferred. Maleic anhydride can be halogen-substituted, e.g., dichloromaleic anhydride and difluoromaleic anhydride.
Functional groups that can participate in cross-linking reactions include ester, alcohol, acid (including carbon-, sulfur-, and phosphorus-based acid) and salt and halide thereof. Other functionalities include anhydride and epoxide. Preferred functional groups include anhydride, especially maleic anhydride. As one skilled in the art will recognize, more than one type of functional group can be present. Normally, however, a single type of functional group is used.
The concentration of functional groups in the fluoropolymer resin component, i.e., in the functionalized fluoropolymer or in functionalized fluoropolymer plus non-functional fluoropolymer, if non-functional fluoropolymer is present, of the melt-fabricable fluoropolymer composition of this invention is effective to achieve cross-linking. As will be recognized by one skilled in the art, the concentration of functional groups that is effective to achieve cross-linking may vary at least with the type of functional group and with the type of cross-linking promoter. The concentration of functional groups present can be expressed relative to the number of main chain carbon atoms in the fluoropolymer resin. Generally, the concentration of functional groups present is at least about 25/106 main chain C atoms, based on total fluoropolymer in the composition. The concentration of functional groups is usually in the range of 25-2500 per 106 main chain C atoms, preferably in the range of 50-2000 per 106 main chain C atoms, based on total fluoropolymer present.
The desired concentration of functional groups in the functionalized fluoropolymer resin can be achieved with a single fluoropolymer having functional groups, or a mixture of such fluoropolymers. The desired concentration of functional groups can also be achieved by blending functionalized fluoropolymer (or mixtures thereof) having a higher concentration of functional groups with non-functional fluoropolymer (or mixtures thereof), i.e. fluoropolymer having essentially no functional groups. In this embodiment, functionalized fluoropolymer acts as a functional group concentrate that can be let down (diluted) with non-functional fluoropolymer. This approach has the advantage of permitting one to achieve a variety of functional group concentrations with a single functionalized fluoropolymer by varying the blending ratio with non-functional fluoropolymer, and is a preferred embodiment of the invention. Preferably, in a functionalized fluoropolymer that is a blend, the functionalized fluoropolymer component is in minor amount relative to non-functional fluoropolymer component.
Thus, in one embodiment of the present invention, the cross-linkable fluoropolymer composition contains minor amounts of functionalized fluoropolymer and a major amount of non-functional fluoropolymer. By xe2x80x9cmajor amountxe2x80x9d is meant at least 50 wt %, preferably at least 70 wt %, of non-functional fluoropolymer based on combined weight of non-functional fluoropolymer and functional fluoropolymer. Thus, in one embodiment of the present invention, the cross-linkable fluoropolymer composition contains minor amounts of functionalized fluoropolymer and a major amount of non-functional fluoropolymer. By xe2x80x9cmajor amountxe2x80x9d is, meant at least 50 wt %, preferably at least 70 wt %, of non-functional fluoropolymer based on combined weight of non-functional fluoropolymer and functional fluoropolymer. In this embodiment of the invention, then, the concentration of functional groups in the functionalized fluoropolymer will be high enough so that the average concentration of functional groups in the functional fluoropolymer plus the non-functional fluoropolymer will be at least about 25/106 main chain C atoms, usually in the range of 25-2500 per 106 main chain C atoms, and preferably in the range of 50-2000 per 106 main chain C atoms.
A wide variety of fluoropolymers can be used. The fluoropolymer is made from at least one fluorine-containing monomer, but may incorporate monomer which contains no fluorine or other halogen. Fluorinated monomers include those which are fluoroolefins containing 2 to 8 carbon atoms and fluorinated vinyl ether (FVE) of the formula CY2xe2x95x90CYOR or CY2xe2x95x90CYORxe2x80x2OR wherein Y is H or F and xe2x80x94R is a completely fluorinated or partially fluorinated linear or branched alkyl group containing 1 to 8 carbon atoms, xe2x80x94Rxe2x80x2xe2x80x94 is a completely fluorinated or partially fluorinated linear or branched alkylene group containing 1 to 8 carbon atoms. Preferred R groups contain 1 to 4 carbon atoms and are preferably perfluorinated. Preferred Rxe2x80x2 groups contain 2 to 4 carbon atoms and are preferably perfluorinated. Hydrocarbon monomers that can be used include ethylene, propylene, n-butylene, and iso-butylene. When the fluoropolymer is functionalized by grafting, preferably at least one monomer contains hydrogen, and in that regard the hydrogen/fluorine atomic ratio in the polymer is preferably at least 0.1/1. The fluoropolymer, however, preferably contains at least 35 wt % fluorine. Fluoropolymer resins that can be used include copolymers of tetrafluoroethylene (TFE) with one or,more copolymerizable monomers chosen from perfluoroolefins having 3-8 carbon atoms and perfluoro(alkyl vinyl ethers) (PAVE) in which the linear or branched alkyl group contains 1-5 carbon atoms. Preferred perfluoropolymers include copolymers of TFE with at least one of hexafluoropropylene (HFP) and PAVE. Preferred comonomers include PAVE in which the alkyl group contains 1-3 carbon atoms, especially 2-3 carbon atoms, i.e. perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinyl ether) (PPVE). Preferred fluoropolymers also include the copolymers of ethylene with perhalogenated monomers such as TFE or chlorotrifluoroethylene (CTFE), such copolymers being often referred to as ETFE and ECTFE, respectively. In the case of ETFE, minor amounts of additional monomer are commonly used to improve properties such as reduced high temperature brittleness. PPVE, PEVE, perfluorobutyl ethylene (PFBE), and hexafluoroisobutylene (HFIB) are preferred additional comonomers. ECTFE may also have additional modifying comonomer. Other fluoropolymers that can be used include vinylidene fluoride (VF2) polymers including homopolymers and copolymers with other perfluoroolefins, particularly HFP and optionally TFE. TFE/HFP copolymer which contains a small amount of VF2, which copolymer is often referred to as THV, can also be used. Examples of perfluorinated copolymers include TFE with HFP and/or PPVE or PEVE. Representative fluoropolymers are described, for example, in ASTM Standard Specifications D-2116, D-3159, and D-3307. Such fluoropolymers are usually partially-crystalline as indicated by a non-zero heat of fusion associated with a melting endotherm as measured by differential scanning calorimetry (DSC) on first melting. Alternatively or additionally, preferred fluoropolymers are non-elastomeric, as opposed to elastomeric.
Functionalized fluoropolymers include fluoropolymers such as those described in the foregoing paragraph and additionally containing copolymerized units derived from functional monomers. If the concentration of functional monomer is high enough in a TFE copolymer, however, no other comonomni mai) be needed. Usually, but not necessarily, the functional groups introduced by such monomers are at the ends of pendant side groups. Examples of functional monomers that introduce pendant side groups having desired functionality include the same ethylenically unsaturated compounds recited above as grafting compounds. Such functional monomers can be incorporated into fluoropolymers, for example, by polymerization in a medium of CO2 as illustrated by example below. Functional monomers that introduce pendant side groups having desired functionality can also have the general formula CY2xe2x95x90CYxe2x80x94Z wherein Y is H or F and Z contains a functional group. Preferably, Y is F and xe2x80x94Z is xe2x80x94Rfxe2x80x94X, wherein Rf is a fluorinated diradical and X is a functional group that may contain CH2 groups. Preferably, Rf is linear or branched perfluoroalkoxy having 2-20 carbon atoms, so that the functional comonomer is a fluorinated vinyl ether. Examples of such fluoropolyether include CF2xe2x95x90CF[OCF2CF(CF3)]mxe2x80x94Oxe2x80x94(CF2)nCH2OH as disclosed in U.S. Pat. No. 4,982,009 and the alcoholic ester CF2xe2x95x90CF[OCF2CF(CF3)]mxe2x80x94Oxe2x80x94(CF2)nxe2x80x94(CH2)pxe2x80x94Oxe2x80x94COR as disclosed in U.S. Pat. No. 5,310,838. Additional fluorovinylethers include CF2xe2x95x90CF[OCF2CF(CF3)]mO(CF2)nCOOH and its carboxylic ester CF2xe2x95x90CF[OCF2CF(CF3)]mO(CF2)nCOOR disclosed in U.S. Pat. No. 4,138,426. In these formulae, m=0-3, n=1-4, p=1-2 and R is methyl or ethyl. Preferred such fluorovinylethers include CF2xe2x95x90CF[OCF2CF(CF3)]O(CF2)2xe2x80x94CH2xe2x80x94OH. These fluorovinylethers are useful because of their ability to incorporate into the polymer and their ability to incorporate functionality into the resultant copolymer. Preferred comonomers that introduce pendant functional groups include maleic anhydride, dichloromaleic anhydride,. difluoromaleic anhydride, and maleic acid.
When functionalized fluoropolymer is achieved by copolymerization, the amount of functional monomer in the functionalized fluoropolymer of this invention is small to achieve the desired concentration of functional groups, even when functionalized fluoropolymer is a blend comprising non-functional fluoropolymer. Generally, the amount of functional monomer is no more than 10 wt %, preferably no more than 5 wt %, based on total weight of functionalized fluoropolymer, i.e., the fluoropolymer component containing the functional monomer. In certain instances, higher concentrations of functional monomer exceeding 10 wt % may be desired, for example, when it is not desired to use a non-functional monomer in the functionalized melt-fabricable fluoropolymer. While the functionalized fluoropolymer can be uniform, it is not necessary to have a uniform concentration of functional-monomer throughout the functionalized fluoropolymer.
When pendant functional groups are introduced into the melt-fabricable fluoropolymer by halogen-free entities, e.g., maleic anhydride or maleic acid, either by grafting or by copolymerizing, the amount of grafting compound grafted to the fluoropolymer or the amount of functional comonomer incorporated into the fluoropolymer will generally be in the range of 0.01-1.0 wt %, preferably 0.02-0.5 wt %, based on total fluoropolymer present in the composition. If the composition contains both non-functional fluoropolymer and functionalized fluoropolymer, the functionalized fluoropolymer will have larger amounts of grafted compound or copolymerized comonomer units depending on the proportion of functionalized fluoropolymer in the composition. Generally, the amount of maleic anhydride or maleic acid is in the range of 0.05 wt % to 5 wt % based on the total weight of functionalized fluoropolymer in such a fluoropolymer blend. Preferably, the amount of maleic anhydride or maleic acid in the functionalized fluoropolymer component of a blend is 0.1-3 wt %, more preferably 0.1-1 wt %.